Piston



SePf- 29, 1942 J. s. DONALDSON 2297,649

PISTON Filed D80. 9, 1969 'l 41 @5 4 L* 4j 50 45 i1 47 5 44 F A 7 I.

45 l 14 4a Z4 205 'g1 zz 2a 46 42 f f 42515 .zo 12 4 47 //'g 4 41 Z22 2@ v 32 v G 41 50 17 4o 72 ZLUEMJM Mw n l [we ATTORNEY Patented Sept. 29, 1942 PISTON John Shearman Donaldson, St. Albans, N. Y., as-

signor to Donaldson Piston Corporation, New York, N. Y., a corporation of New Jersey Application December 9, 1939, Serial No. 308,327

10 Claims.

This invention relates to pistons, and more especially to pistons for internal combustion engines. The invention aims generally to provide an improved piston which shall be highly eicient, oi low oil consumption, long wearing, easily and quickly installed and removed, and of comparatively low manufacturing cost.

This application is a continuation in part of my former application Serial No. 51,782, filed November 27, 1934.

Pistons for internal combustion engines as commonly made comprise a ring-carrying head portion at the combustion end of the piston, a skirt portion, and means for connecting the pist0n to the upper end of the connecting rod. 'I'he head portion is guided and held in alignment by the skirt as the piston reciprocates in the cylinder. The skirt must, therefore, be rigid with the head portion of the piston, and should fit as accurately and closely to the cylinder wall as possible without interfering with its sliding movement under all engine operating temperatures.

Cylinders of internal combustion engines are usually made of cast iron and usually either water-cooled or air-cooled. Pistons are not usually either water-cooled or air-cooled, and in order to make them as light as possible the pistons are commonly made of a metal or alloy which is lighter or stronger than cast iron, but which usually also has a higher coeicient of expansion than cast iron. The transfer of heat during engine operation from the piston to the cooled cylinder wall limits to some extent the maximum temperature of the piston skirt and resulting expansion. However, because of the construction w of the piston and the high maximum temperature of the skirt, it has been necessary heretofore in iitting the pistons to allow for a greater eX- pansion of the piston skirt than that of the cylinder wall by making the skirt of a diameter suiciently less than the internal diameter of the cylinder that in operation of the engine the skirt will not, due to its greater expansion, iit so tight in the cylinder as to seize or become frozen thereto. The diiiiculty due to expansion of the piston skirt has been increased by the fact that for the commonly used wrist-pin connection between the piston and the connecting rod heavy bosses are provided on opposite sides of the piston for journaling the wrist pin, and these relatively heavy masses of metal conducting heat from the head cause unequal expansion and contraction at different parts of the skirt.

It has been the customary practice, therefore,

in iitting cast iron pistons of the so-called solid skirt type, that is, having a circumferentially continuous skirt, to lit them with a clearance of .001 of an inch for each inch of diameter; that is, to make the skirt when cold smaller than the cylinder bore by an amount equal to .001 of an inch for each inch of diameter of the bore. Solid skirt pistons of aluminum alloy when used have usually been itted with a clearance of .002 of an inch for each inch of diameter.

In order to avoid such large clearances, pistons made of aluminum alloy and other alloys h-aving relatively high coefficients of expansion, are usually made with the skirt slotted diagonally down one side. Such split skirts, however, have the disadvantage that they do not hold the ringcarrying head of the piston as rigidly as do solid skirts, and that as the result of use this disadvantage increases, and, in addition, such split skirts frequently collapse after more or less long use due to metal fatigue.

The present invention aims to avoid the necessity of large clearances between the piston skirt and the cylinder and of slotting the skirt, and to provide a light-weight piston having a circumferentially continuous, or solid, skirt which may be fitted more closely than has heretofore been found possible; to provide an improved ball-andsocket connecting means between the piston and the connecting rod; and to provide a piston of which the head, including the part carrying the connecting rod bearing, may be die-cast of comparatively light-weight metal, and the skirt may be made of thin, rolled or otherwise worked, comparatively hard and fine-grained wear-resisting metal, or other suitable strong wear-resisting material which will take a smooth lowfriction surface.

To these ends, a piston embodying the features of the invention in the form no-w considered best, has its connecting rod bearings carried by the head portion independently of the skirt, and has a tubular skirt of thin hard Wear-resisting material, preferably a suitable steel or steel alloy, the coeflicient of expansion of which is not much greater and may be somewhat less than that of the cylinder, and the skirt is spaced radially from the bearing part of the head and rigidly connected to the ring-carrying part of the head by three or more symmetrically arranged connecting members cast into the head and welded to the skirt. The skirt is most desirably skeletonized by having symmetrically arranged openings formed therein, and for greater rigidity it has an inwardly extending flange at its lower edge, and as an additional means for more securely holding the head and skirt in axial alignment the skirt is braced by three or more legs extending between a l-ower part of the bearing part of the head and points near the bottom of the skirt.

Instead of the more commonly used wrist pin connection between the piston and the connecting rod, the connection is most desirably of the balland-socket type, a ball on the end of the connecting rod being seated between upper and lower bearing members carried by a downwardly extending central tubular body portion of the piston head by which heat is conducted away from the top part of the head and dissipated to the splashed up crank case oil; and a feature of the present invention lies in the means, hereinafter described, whereby the upper bearing member is held in adjusted position, removable from the top of the piston head, and the removable holding means is locked and sealed against entrance of carbon and loss of compression; and another feature of the invention is found in the means provided for lubricating the bearing surfaces between the connecting rod ball and the bearing members.

Because of the manner in which the skirt is connected to the head portion of the piston, the conduction of heat from the head to the skirt is largely reduced; and because of the free access of splashed crank case oil to the inside of the skirt, a comparatively good cooling of the skirt is secured. In addition tothis, heat-developing friction between the skirt and the cylinder wall is reduced by the skeletonizing of the skirt and the resulting increase in oil movement between the skirt surface areas and the cylinder wall, which both reduces the friction and increases the cooling; and the connecting members between the skirt and the head portion of the piston are also cooled by the splashed oil. The maximum operating temperature of the skirt is thus reduced; and because of this together with its comparatively low coefficient of expansion, the skirt may be fitted with a much closer clearance than has heretofore been found practical with the socalled solid skirts. It has been found that pistons according to the invention having plain carbon steel skirts about of an inch thick may be tted with a clearance of less than .09025 of an inch for each inch of diameter without causing any troub-le as the result of skirt expansion. Nor have such skirts shown any lack of rigidity in operation.

The hard dense wear-resisting metals of which the skirt is most desirably made are comparatively heavy, but they are also correspondingly strong, and because of this, and of the fact that the skirt is free from operating strains and stresses set up through the connection between the piston and the connecting rod, the skirt can safely be comparatively thin and skeletonized, and, therefore, comparatively light. The head portion of the piston, including the bearings-carrying part, may be, and most desirably is, made of a light weight alloy so that the piston as a whole will be of exceptional lightness.

The invention thus provides a light weight piston having a solid skirt which may be exceptionally close fitting and of great strength, lightness and wear-resistance and with a smooth lowfriction surface.

Because of the close t of the piston skirt in the cylinder bore and its smooth surface and wear-resistance, and the comparatively light weight of the piston as a whole, a much improved and more efcient operation of the engine through a long period of use is obtained. The ring-carrying part of the piston is accurately guided, and cocking of the piston with resulting piston slap and uneven wear and rounding of the piston rings and resulting passing of oil and loss of vacuum and compression and power are almost, if not completely, eliminated. It is well known that with the pistons commonly in use there is a comparatively great loss of engine power due to the cooking of the pistons and the resulting rounding of the rings, and that this loss of power is aggravated by the passing of oil which in turn results in carbon formation, causing further loss of power and in extreme cases preignition.

A full understanding of the invention can best be given by a ydetail description of a piston embodying the various features of the invention in the form now considered best, and such a description will now be given in connection with the accompanying drawing showing such a piston.

In said drawing:

Fig. l is a View of the piston in side elevation;

Fig. 2 is a sectional view taken on line 2 2 of Fig. l;

Fig. 3 is a sectional View looking into the piston from the bottom, or skirt end, taken on line 3 3 of Fig. 2;

Fig. 4 is a front view, Fig. 5 an edge View, and Fig. 6 a top end view, of one of the connecting members by which the skirt is secured to the head of the piston;

Fig. '7 is an enlarged fragmentary sectional view which will be hereinafter explained;

Fig. 8 is a fragmentary side view showing a modification of the means for locking the skirt to a connecting member;

Figs. 9 and l0 are sectional views taken on lines 9 9 and [iB l of Fig. 8;

Figs. l1 and l2 are views similar respectively to Figs. 9 and l0 but showing the locking lugs efore being bent to locking position; and

Fig. 13 is a fragmentary sectional View taken on line 2 2 of l but showing a modified form of the upper of the two members which provide the spherical bearing for the ball on the end of the connecting rod.

Referring to the drawing, the head portion of the piston comprises an annular top part I9 having a ring-carrying apron or part Il, and a central downwardly extending body portion or part l2 which, when the piston is to be connected to the connecting rod by a ball and socket joint, as in the example shown, is a tubular part the bore of which extends upward through the top of the piston head. This tubular body part l2 is the bearing-carrying part of the piston. It has at the bottom an inside shoulder l5 for supporting an annular lower bearing member I6 the inner bearing surface of which is spherically curved to fit the ball il on the end of a connecting rod l to which the piston is to be connected. The opening in the bottom ofthe bearing member is of suicient size as compared to the greatest diameter of the piston rod just below the ball to permit the necessary oscillation of the piston rod. The bore of the tubular part l2 is threaded to receive a threaded upper bearing member 2Q which has a spherically curved bearing surface to engage an upper half of the connecting rod ball when the lower end of the member is seated on the lower bearing member, except for a central oil recess or pocket 2i opening upward from the bearing surface. The bearing member 2i! has a plain top surface and is of such depth or vertical length than when screwed down into position its upper surface is ush with the surface of the top part lll or substantially so, and as shown in Fig. 2 is made of a single integral piece of metal.

An annular portion or Zone 22 of the peripheral surface of the bearing member 20 is unthreaded, and a corresponding annular portion or zone 23 of the bore of the tubular part l2 into which the bearing member is screwed is also unthreaded, and these smooth annular faces 22 and 23 are of such relative size and extend in such direction axially that when the upper bearing member is screwed into the bore of the tubular part l2 these annular surfaces 22 and 23 come into engagement somewhat before the upper bearing member has been screwed in to its seated position, and that, as the upper bearing member is thereafter screwed further in to seat against the lower bearing member, the two coacting surfaces are wedged tightly together with a force sufficient to slightly upset the adjacent metal of one or both of the engaging bodies, thus securely locking the two bearing members in position in the tubular part I2 and forming a seal against loss of compression and against any possible introduction of carbon from the top of the piston to the connecting rod bearing surfaces.

Most desirably, the wedging surface 22 of the bearing member 2i! is slightly tapered down- Wardly and the corresponding wedging surface 23 of the bore is tapered downwardly and inwardly at an angle slightly less than the angle of the wedging surface of the bearing member,

the relative diameters of these two coacting wedging surfaces being such that the tapered face 22 of the bearing member comes into engagement with the upper part of the tapered face 23 of the bore, as illustrated by Fig. 4, somewhat before the upper bearing member has been ,l

screwed down far enough to become seated on the lower bearing member. As the upper bearing member is then further screwed down, the wedging action of its tapered face 22 against the tapered face 23 of the piston head causes a slight yielding of the metal until when the upper bearing member has been screwed down to its seated position the faces 22 and 23 are so tightly wedged together that the upper bearing member is thereby securely locked and held against any unintended loosening as the result of vibration, and a seal against loss of compression or possible introduction of carbon is obtained.

The wedging surfaces 22 and 23 are most desirably formed at the top of the bearing member and piston head as shown. This gives better opportunity for slight upsetting of the metal under the wedging force, and the sealing of the surfaces in this position protects all parts of the threading for the bearing member from access of carbon. The exact degree of the slight taper of the faces 22 and 23 in the preferred form of the locking and sealing means and the difference in the taper of the two faces which will give best results will depend on the metal of which the bearing member and head are made. AAn angle of 5 degrees for the taper of the face 22 and an angle of 4 degrees for the face 23 have been found most suitable when the bearing member and piston head are made of a magnesium alloy.

Instead of being made of a single piece of metal as shown in Fig. 2, the upper bearing member may be made of two separate pieces as shown in Fig. 13, that is, a lower piece 20a which seats on the lower bearing member I6 and which need not be threaded, and an upper piece Zlib which seats on the piece 22a and which is screwthreaded except for its annular unthreaded wedging surface 22.

In order to give wrench hold, two wrench pin sockets 24 are provided in the top face of the upper bearing member; and to hold the piston against turning while the upper bearing member is being turned to remove it or to screw it down into position, the top part l0 is also provided with wrench recesses 25, preferably three, of which one is shown in Fig. 2.

The ball l'l is detachably secured on the end of the connecting rod I8 by the means disclosed and claimed in the Donaldson Re. Patent No. 19,904, a slightly reduced end 25 of the rod being threaded to screw into a threaded bore extending most of the way through the ball,

and the ball being screwed on to the threaded end of the rod until it seats against the shoulder 21 at the end of the threaded part of the rod, and the ball being locked against unintended unscrewing from the rod by a locking screw 2B which extends through a countersunk opening in the ball and is screwed into a threaded bore in the end of the connecting rod. The external and internal threads on the end of the connecting rod are respectively one a right hand thread and the other a left hand thread, so 'that when the ball has been screwed down `against the shoulder 2l and the locking screw screwed in until its head seats against the shoulder 29 of the screw opening of the ball, the ball will be securely locked against any unintended unscrewing turning movement.

For lubricating the ball-and-socket connecting rod bearing, a plurality of oil passages 3i! are formed in the wall of the tubular body part i2, one of which is shown in Fig. 2. These passages lead to an annular channel 3l formed by matched annular grooves in the threading of the bore of the tubular part I2 and in the threading of the upper bearing member 20, or by an annular groove in. one of these members, I2 or 253, as in Fig. 13, and from this channel 3l a plurality of passages 32 formed in the bearing member, of which one is shown in Fig. 2, lead to the oil recess or pocket 2 l The passages 32 may or may not register with the passages 3E! according to the angular position bf the bearing member when it is seated against the lower bearing member. Crank case oil splashed up within the piston and between the ringcarrying part Il and the tubular body part I2 will enter the passages 3c and by passing through the channel 3l and passages 32 reach the oil pocket 2l. Then, as the connecting rod rocks in reciprocating the piston, oil from the pocket 2l at the top of the ball will on the up strokes of the piston be forced down about the ball and becomes distributed between the bearing surfaces of the ball and the bearing members, finally working out from the bottom edge of the lower bearing member. The head of the locking screw 28 most desirably lies slightly below the adjacent surface of the ball, forming a depression which aids in starting the oil between the bearing surfaces as the ball rocks.

The skirt d@ consists of a short piece of round thin-wall tubing of suitable hard, wear-resisting, smooth-surface material, preferably steel o1' a suitable steel alloy or other metal alloy having a coeflicient of thermal expansion not substantially greater than that of metal of the cylinder in which the piston is to be used. I have used, and found satisfactory, rolled seamless tubing of a medium carbon steel of approximately inch wall thickness for pistons of about 3 inch diameter. Such steel has a coecient of expansion only slightly greater than that of cast iron of which the cylinder blocks of internal combustion engines are usually made. Instead of making the skirt of seamless tubing, welded tubing may be used made of rolled sheet steel or other suitable metal. However formed, it is desirable that the skirt be of rolled or otherwise worked metal, or of metal having characteristics of worked metal, such as centrifugally cast tubing, rather than of ordinary cast metal because of the greater density and strength of worked metal.

The skirt is secured to the head portion of the piston by three or more connecting members di extending between the ring-carrying part Ii and the skirt. These connecting members are most dsslrably, and as shown, formed separately from the head and skirt of steel or other suitable strong metal cast into the ring-carrying part II of the head in position to extend downward within the skirt to lie against the inner surface of the skirt, to which they are securely welded. At the points where the connecting members il are embedded in the head, the ringcarrying part `and the annular top part lo of the head arethickened as indicated at 62 in Fig. 2 and as shown by Fig. 3, and the wrench pin sockets in the upper face of the part l) are located at these thickened parts.

rihe connecting members di are most desirably stamped from sheet metal and formed to the shape shown. For more securely anchoring the members in the cast piston head, the upper end portion of the member includes a part which when the member is embedded in the piston head lies just back of the lower ring groove extending parallel to the axis of the piston and a bent-over end portion l extending inward, and the edges of these parts are indented and the part L35 formed with an opening into which indent-ations and openings the cast metal of the head extends. The turned-over end 46 also has a central perforation therein which is of greater diameter than the wrench sockets 25 so that it serves to permit the entrance of metal for l Below the part lo the connecting member has an outwardly inclined portion 4l which may have an opening 48 therein and then a lower end portion 49 extending parallel to the part l5 and which in the assembled piston lies against and is welded to the skirt. The connecting members because of their small cross-section and of their being exposed to and cooled by the splashed oil conduct comparatively little heat from the piston head tc the skirt. In order to minimize such heat conduction to the greatest possible extent consistent with the necessary strength, the connecting members are most desirably made of a suitable strong alloy oflow thermal conductivity.

In addition to the welding of the connecting member 4i to the skirt, or, as it may be, in place of welding, the connecting members and the skirt may be mechanically locked against relative edgewise movement. As shown in Figs. 1 to 6, this mechanical locking of the connecting members to the skirt is eiected by a lug 5S projecting from the outer surface of the lower portion 49 of the connecting member and extending into and fitting closely in an opening in the skirt. The lug may be punched into the opening after the parts are assembled, or it may be formed at the time the connecting member is cut and shaped, and in such case in assembling the skirt with the head and connecting members the skirt is sprung slightly out of round in order to pass its upper edge over the lugs. Obviously, locking lugs might be formed to extend from the skirt into openings in the portions #i9 of the connecting members.

The skirt 40 is most desirably skeletonized by having symmetrically arranged openings formed therein as shown by Figs. 1 and 2, a circumferentially continuous band being left at the top and bottom of the skirt. The skirt is thus made lighter, and friction between the skirt and the cylinder wall is reduced by lessening the extent of surface of the skirt and by obtaining an increased oil movement between the skirt and the cylinder wall. The increased access and movement of oil also increases the cooling of the skirt. Such a skeletonized skirt is more rigid than a thinner unskeletonized skirt of equal weight, and has all the advantages of an unskeletonized skirt as a guide for the head portion of the piston. 'Ihe symmetrically arranged openings in the skirt are most desirably, as in the construction shown, shaped and arranged so that the connecting strips of metal between the openings shall be inclined alternately in opposite directions. These inclined wall strips have the advantage of causing a better distribution of the oil between the skirt and the cylinder wall. The disposition of the openings should also be, as shown, such as to leave suicient portions of the skirt wall adjacent the points of attachment of the connecting members 4I to avoid undue weakening of the skirt at these points of greatest strain. In order to increase the rigidity of the skirt and permit it to be made thinner than would otherwise be suitable, the skirt is most desirably formed with an inwardly extending ange 55 at its lower edge.

Instead of depending solely on the connecting members for holding the head and skirt in axial alignment, the upper end of the skirt most desirably extends upward to engage the ring-carrying part I I, and, in addition, the skirt is most desirably braced by three or more legs 56 extending between a lower Dart of the central tubular body part I2 and points near the bottom of the skirt. Such bracing legs are provided in the construction shown by a spider cut and shaped from sheet steel or other strong metal and comprising a collar 5l which is sleeved on the reduced lower end of the central tubular part I2 and three radially and downwardly extending legs the outer ends oi which are bent down to extend parallel to the skirt and are welded to the skirt at points near the bottom thereof. The legs 56 serve merely to brace the skirt in radial directions and do not need to be rigidly connected to or have an absolutely close fit to the tubular part I I The collar 5l is therefore fitted to slide easily over the lower end of the part I2 for convenience in assembly and to avoid straining the collar beyond its elastic limit and possible fracture by expansion of the part I2 which is of metal having a higher coeiicient of expansion than the metal of which the spider is formed.

An alternative means for mechanically locking the skirt to the connecting member 4I is illustrated by Figs. 8 to 12. As shown in these figures, the connecting members are of such length that when the skirt is positioned the connecting members extend down just to the edges lil of the skirt wall at the top of openings which are just below the parts of the skirt wall to which the connecting members are attached, and the skirt has extending downward from each edge lll two spaced lugs ll, and each connecting member has extending from its lower edge a centrally located lug '12. When the parts are assembled, the lugs 'H are bent inward beneath and against the lower edge 0f the connecting membersy as shown by Fig. 9, and the lugs 'l2 of the connecting members are bent outward to lie between the lugs 1I and against the skirt wall edge l), as shown by Fig. l0. The lugs 'i l Will thus prevent any relative movement between the connecting members and the skirt in one axial direction, and the lugs l2 will prevent any such relative movement in the other axial direction; and by making the lugs 'il and l2 of suitable spacing and width they serve also to prevent relative circumferential movement. The connecting members and the skirt will thus be positively held by these locking lugs against relative edgewise movement.

What is claimed is:

l. A piston, comprising a head portion having a ring-carrying part and a downwardly extending central tubular part the diameter of which is substantially less than the inside diameter of the skirt and which is provided with bearing members forming the socket portion of a balland-socket joint for a, connecting rod having a ball head, and a circumferentially continuous skirt welded to connecting members extending downward from said ring-carrying part.

2. A piston, comprising a skirt, and a head portion having a ring-carrying part and a downwardly extending central tubular part the diameter of which is substantially less than the inside diameter of the skirt and which is provided with upper and lower bearing members for the ball head of a connecting rod, the lower bearing member being annular and having a bearing surface spherically curved to lit the ball, and the upper bearing member having a spherically curved bearing surface to t an upper half of the ball and having a central oil pocket opening upward from its bearing surface, the bore of said tubular part being screw-threaded and the upper bearing member being threaded and screwed into said threaded bore, the threaded face of one of said last mentioned elements having a circumferential groove therein, the wall of the tubular part having an oil passage therethrough to the channel formed by said groove, and the upper bearing member having an oil passage leading from said channel to said oil pocket.

3. A piston, comprising a skirt, and a head portion having a ring-carrying part and a downwardly extending central tubular part the diameter of which is substantially less than the inside diameter of the skirt and which is provided with upper and lower bearing members for the ball head of a connecting rod, the lower bearing member being annular and having a bearing surface spherically curved to fit the ball, and the upper bearing member having a spherically curved bearing surface to fit an upper half of the ball and having a central oil pocket opening upward from its bearing surface, the bere of said tubular part being screw-threaded and the upper bearing member being threaded and screwed into said threaded bore and seated on the lower bearing member, the threaded faces of the upper bearing member andV the bore having matching grooves forming an annular oil channel, the wall of the tubular part having oil passages therethrough to said channel, and the upper bearing member having oil passages leading from said channel to said oil-pocket.

4. A piston, comprising a skirt, and a head portion having a ring-carrying part and a down-- wardly extending central tubular part the diameter of which is substantially less than the inside diameter of the skirt, and upper and lower bearing members for the ball head of a connecting rod fitted in said tubular part, the lower bearing member being annular and having a bearing surface spherically curved to t the ball, and the upper bearing member having a spherically curved bearing face to fit an upper half of the ball and having a central oil pocket opening upward from the bearing face, an annular oil channel between the upper bearing member and the wall of said tubular part formed by a circumferential groove in one of saidelements.

5. A piston, comprising a head portion having a ring-carrying part and a downwardly extending central tubular part the diameter of which is substantially less than the inside diameter `of the skirt of the piston, and a skirt rigidly connected to said ring-carrying part, said tubular part being provided with upper and lower bearing members for the ball head of a connecting rod, the lower bearing member being annular and having a bearing surface spherically curved to fit the ball, and the upper bearing member having a spherically curved bearing surface to t an upper half -of the ball, the bore of said tubular part being screw-threaded and the upper bearing member being threaded and screwed into the threaded bore, the upper bearing member having a smooth surfaced annular portion slightly tapered downwardly and the bore of said tubular part having a coacting smooth surfaced annular portion tapered downwardly and inwardly at an angle slightly less than that of the tapered portion of the upper bearing member, the relative diameters of the two coacting tapered portions being such that the tapered portion of the bearing member will come into engagement with the upper part of the tapered portion of the tubular member before the upper bearing member has been screwed into its position of adjustment.

6. A piston, having a central bore through its head portion screw-threaded in its upper part and having upper and lower bearing members for the ball head of a connecting rod in said bore, the lower bearing member being annular and having a bearing surface spherically curved to fit the ball and being seated in the lower part of said bere, the upper bearing member having a spherically curved bearing face and having a portion of its peripheral surface threaded and being screwed into the threaded portion of the bore and seated on the lower bearing member, the upper bearing member having also an annular unthreaded wedging surface, and the bore having a coacting annular unthreaded wedging surface, the relative diameters of said Wedging surfaces being such that they come into engagement before the upper bearing member is fully seated and that when the upper bearing is screwed further in to seat on the lower bearing member the wedging surfaces are wedged tightly together with a slight upsetting of adjacent metal.

9. A piston as claimed in claim 6, in which the upper bearing member is made of a single integral piece of metal.

10. A piston as claimed in claim 6, in which the upper bearing member is made of two separate pieces, the lower of which seats on the lower bearing member and the upper of which seats on the lower piece and is screw-threaded and has the annular wedging surface.

JOHN SHEARMAN DONALDSON. 

